Pulse tube refrigerator and method of operating thereof

ABSTRACT

A pulse tube refrigerator includes a compressor, a pulse tube including an internal space, a regenerator loaded with a regenerative material for exchanging heat with working gas, a buffer including an internal space with a predetermined capacity, a flow passage to connect an end of the pulse tube and the buffer, a temperature detecting section to detect temperature of the working gas, a flow adjusting section to adjust an amount of flow of the working gas flowing through the flow passage, and a controlling section to control the flow adjusting section in response to the temperature of the working gas detected at the temperature detecting section.

RELATED APPLICATION

Priority is claimed to Japanese Priority Application No. 2012-063188, filed on Mar. 21, 2012, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.

BACKGROUND

1. Technical Field

The invention relates to a pulse tube refrigerator and a method of operation thereof.

2. Description of Related Art

For example, a pulse tube refrigerator has a compressor, a pulse tube, a regenerator, a phase control mechanism, and the like. High-pressure working gas generated in the compressor passes through the regenerator and the pulse tube, then flows into the phase control mechanism.

The phase control mechanism is configured with a buffer and an inertance tube which is disposed between the pulse tube and the buffer. The phase control mechanism generates a phase difference between varying pressure and varying flow of the working gas oscillating like sine waves, supplied from the compressor in the pulse tube. Thus, a cold thermal mass is generated between the pulse tube and the regenerator.

SUMMARY OF THE INVENTION

According to at least one embodiment of the present invention, a pulse tube refrigerator includes a compressor, a pulse tube including an internal space, a regenerator loaded with a regenerative material for exchanging heat with working gas, a buffer including an internal space with a predetermined capacity, a flow passage to connect an end of the pulse tube and the buffer, a temperature detecting section to detect temperature of the working gas, a flow adjusting section to adjust an amount of flow of the working gas flowing through the flow passage, and a controlling section to control the flow adjusting section in response to the temperature of the working gas detected at the temperature detecting section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a pulse tube refrigerator according to an embodiment of the present invention;

FIG. 2 is a flowchart showing a controlling procedure executed by a controller;

FIG. 3 is a graph showing a relationship between a characteristic of an inertance tube and cool-down time; and

FIG. 4 is a graph showing a relationship between a characteristic of an inertance tube and cool-down time.

DETAILED DESCRIPTION

The invention will now be described by reference to the preferred embodiments. This does not intend to limit the scope of the present invention, but to exemplify the invention.

Characteristics of an inertance tube, which is a part of the phase control mechanism, are set to obtain the maximum refrigeration capacity at a specified refrigeration temperature of a pulse tube refrigerator. Therefore, even while a refrigerator is starting up, i.e., the temperature of the working gas is still high, an amount of flow of the working gas flowing through the inertance tube is set to the amount of flow corresponding with the specified refrigeration temperature of the refrigerator. Therefore, with a conventional pulse tube refrigerator, there is a problem that it takes a long cool-down time which is a time needed to reach the specified refrigeration temperature of the refrigerator from a normal temperature.

FIG. 1 shows a regenerative refrigerator according to the first embodiment of the present invention. In the present embodiment, a Stirling pulse-tube refrigerator 1 (called simply a “refrigerator”, hereafter) is taken as an example of a regenerative refrigerator to be explained. The refrigerator 1 has, on the whole, a compressor 2, an extender 3, and a phase control section 4.

The compressor 2 is configured with a cylinder 6, pistons 7, linear motors 8, plate spring units 15, and the like in a housing 5.

The cylinder 6 is disposed at the center of the housing 5, extended in the horizontal direction in FIG. 1. In the cylinder 6, a pair of pistons 7 are disposed, facing to each other. The pistons 7 in the cylinder 6 are configured to be capable of making a reciprocating motion in the axial direction (the horizontal direction in FIG. 1). In between the pair of the pistons 7, a compressing chamber 12 is formed. The compressing chamber 12 communicates with the expander 3 via a passage 13.

A linear motor 8 is provided for each of the pistons 7. The linear motor 8 drives the piston to make a reciprocating motion in the cylinder 6. The linear motor 8 is configured with a permanent magnet 9, an electromagnetic coil 10, a yoke 11, and a support holder 19.

The permanent magnet 9 is fixed to the piston 7 by the support holder 19. Therefore, the permanent magnet 9 moves in conjunction with the piston 7. Also, the yoke 11 is fixed to the housing 5. A ring-shaped concave section is formed on the yoke 11, to make the permanent magnet 9 movable in the axial direction in the concave section.

The electromagnetic coil 10 is fixed at a position opposite to the permanent magnet 9 in the concave section of the yoke 11. Alternating current oscillating with a prescribed frequency is supplied to the electromagnetic coil 10 from a power source (not shown). Once the alternating current is supplied to the electromagnetic coil 10, a driving force is generated between the permanent magnet 9 and the electromagnetic coil 10 in the axial direction. As mentioned earlier, since the electromagnetic coil 10 is fixed on the yoke 11, the piston 7 is driven in the cylinder 6 in the axial direction by the driving force generated with the linear motor 8.

The plate spring unit 15 has its external circumference fixed to the housing 5 via the support member 14, as well as having its internal circumference fixed to the piston 7. The plate spring unit 15 has a function to support the piston 7 to make reciprocating motion in the compressor 2. Therefore, when the piston 7 is driven in the axial direction by the linear motor 8, the plate spring unit 15 allows the piston 7 to move in the axial direction, and after the piston 7 has moved, energizes the piston 7 with elastic repulsive force directed toward the opposite direction to the direction driven with the linear motor 8.

Thus, each of the pistons 7 reciprocates in the axial direction in the cylinder 6, to oscillate pressure of the working gas in the compressing chamber 12. The varying pressure of the working gas in the compressing chamber 12 is supplied to the expander 3 via the passage 13, to generate a cold thermal mass in the expander 3.

The expander 3 has a regenerator 20, a pulse tube 21, a low-temperature heat exchanger 22, and the like, to be configured in a pulse tube refrigerator.

The regenerator 20 is disposed in the middle of a flow passage of the working gas flowing from the compressor 2 to the pulse tube 21. The regenerator 20 is configured to have loaded a regenerative material to accumulate cold thermal energy in its inner part of a cylindrical body.

The pulse tube 21 is a cylindrical tube, communicating with the regenerator 20 via a passage 22 a provided in the low-temperature heat exchanger 22. It is noted that although in the present embodiment, the regenerator 20 and the pulse tube 21 are connected with a folded connection, it is possible to adopt an in-line connection.

Next, operations of the pulse tube refrigerator 1 will be explained. Energy of the working gas supplied by the compressor 2 is transferred through the regenerator 20, the low-temperature heat exchanger 22, and the pulse tube 21 in order, to be consumed at the phase control section 4. Between the regenerator 20 and the pulse tube 21, an energy gap is generated due to work done when the working gas having the generated phase difference transitions from an isothermal state to an adiabatic state. To compensate for the gap, heat is absorbed at the low-temperature heat exchanger 22, which generates a cold thermal mass. On the other hand, at a radiator 23 disposed at the higher temperature side of the pulse tube 21 (the lower end of the pulse tube in FIG. 1), the heat absorbed at the low-temperature heat exchanger 22 is radiated. By repeating the series of operations, an object to be cooled, thermally connected to the low-temperature heat exchanger 22, is cooled.

Next, the phase control mechanism 4 will be explained. The phase control mechanism 4 is configured with a flow passage 24, a valve device 27, a thermometer 28, a buffer tank 29, a controller 30, and the like.

The flow passage 24 connects a high-temperature end of the pulse tube 21 and the buffer tank 29 having an internal space with a prescribed capacity. In the present embodiment, the flow passage 24 is configured with the inertance tube 25 (corresponding to a main flow passage in claims) and a bypass tube 26 (corresponding to a bypass flow passage in claims), which is separate from the inertance tube 25.

The inertance tube 25 has an appropriate tube length and a diameter to flow the working gas at a specified refrigeration temperature of the refrigerator 1, for example, 77 K. On the other hand, the bypass tube 26 has a smaller diameter than the pulse tube 21. In the present embodiment, the inertance tube 25 and the bypass tube 26 has the same tube length.

The valve device 27 (corresponding to a flow adjusting section in the claims) is attached to the bypass tube 26. The valve device 27 is an electromagnetic valve driven and controlled by the controller 30, to control the amount of flow of the working gas in the bypass tube 26. An amount of opening of the valve device 27 can be adjusted continuously.

The thermometer 28 (corresponding to a temperature detecting section in the claims) is attached to the low-temperature heat exchanger 22. The thermometer 28 measures a temperature at a low-temperature end of the pulse tube 21, which is equivalent to the temperature of the working gas flowing in the passage 22 a. Therefore, it is possible to determine whether the temperature of the low-temperature heat exchanger 22 reaches the specified refrigeration temperature by measuring with the thermometer 28. The temperature of working gas measured with the thermometer 28 is sent to the controller 30.

The controller 30 controls the valve device 27 in response to the temperature of working gas measured with the thermometer 28. Therefore, the amount of flow of the working gas flowing between the pulse tube 21 and the regenerator 20 is controlled by the controller 30. Specifically, if the valve device 27 is closed completely, the amount of flow of the working gas flowing through the flow passage 24 flows only through the inertance tube 25, which is the optimum flow in terms of maintaining the specified refrigeration temperature (for example, 77 K) of the refrigerator 1. On the other hand, if the valve device 27 is fully opened, the working gas flowing through the flow passage 24 flows through both the inertance tube 25 and the bypass tube 26, with which the amount of flow is larger than the amount obtained only with the inertance tube 25.

The phase control mechanism 4 configured as above is to generate work of flow in the pulse tube 21, and has a function to delay change of displacement of the working gas behind change of pressure of the working gas, where both the displacement and pressure are oscillating. This phase delay generated by the phase control mechanism generates the work of flow from the compressor 2 (source of oscillation), which generates a cold thermal mass at the low-temperature end of the pulse tube 21.

The phase control mechanism 4 and the pulse tube 21 can be explained in terms of electrical circuitry, where the pulse tube 21 and the buffer tank 29 correspond to capacitor components, the flow passage 24 corresponds to an inductance component and a resistance component. Therefore, by adjusting characteristics of the flow passage 24, in other words, by adjusting the amount of flow of the working gas flowing through the inertance tube 25 and the bypass tube 26, it is possible to adjust the phase difference.

FIG. 2 is a flowchart showing a controlling procedure of the valve device 27 executed by the controller 30, which is one of methods of operating the refrigerator 1. The procedure shown in FIG. 2 is executed routinely, for example, every predetermined period.

Once the procedure shown in FIG. 2 starts, the controller 30 first determines whether the refrigerator 1 is in a start-up state at Step S10.

If it is determined that the refrigerator 1 is in the start-up state at Step S10, it means that the working gas has not yet been cooled down. Therefore if a positive determination is made at Step S10 (YES), the procedure proceeds to Step S11, where the controller 30 opens the valve device 27 fully. After Step S11, the procedure proceeds to Step S12. On the other hand, if it is not determined that the refrigerator 1 is in the start-up state at Step S10, the procedure proceeds to Step S12 without branching to Step S11.

At Step S12, the controller 30, based on a signal sent from the thermometer 28, reads a current temperature X of the working gas. The read temperature X of the working gas is sent to the controller 30.

Next, at Step S13, the controller 30 determines an amount of opening of the valve device in response to the temperature X of the working gas read at Step S12.

Specifically, the controller 30 has a two dimensional map that has two parameters, where one is a changing temperature of the working gas and the other is an amount of opening of the valve device 27, which is stored in the controller 30 beforehand. The controller 30 also stores a previous temperature that was obtained when the procedure was executed last time, which is compared with the current temperature X of the working gas to obtain a change in the temperature.

If the controller 30, based on the obtained change, determines that the temperature of the working gas is rising, the valve device 27 is opened by a predetermined amount of opening. If the valve device 27 is already fully opened, it is maintained. With this operation, the diameter of the bypass tube is widened (flow resistance is reduced), which increases the amount of flow of the working gas through the bypass tube 26. If the valve device 27 is already opened fully, the maximum amount of flow is kept flowing.

On the other hand, if the controller 30, based on the obtained change, determines that the temperature of the working gas is falling, the controller 30 closes the valve device 27 by a predetermined amount. With this operation, the diameter of the bypass tube 26 is throttled down (flow resistance is increased), which reduces the amount of flow of the working gas through the bypass tube 26. When the temperature X of the working gas reaches the specified refrigeration temperature, the valve device 27 becomes closed completely.

If the valve device 27 is closed completely, the working gas flows only through the inertance tube 25 between the pulse tube 21 and the buffer tank 29. As described earlier, the inertance tube 25 is set to operate optimally at the specified refrigeration temperature. Therefore, at a joining section of the regenerator 20 and the pulse tube 21, a cold thermal mass can be generated efficiently.

It is noted that an amount of opening of the valve device 27, which is adjusted in response to a change of the temperature, depends on the specified refrigeration temperature of the refrigerator 1, diameter of the bypass tube 26, length of the flow passage 24, or the like. The map stored in the controller 30 is generated with an experiment or other means which takes these factors into account.

In the refrigerator 1 according to the present embodiment, since the controller 30 controls the valve device 27 as above, the valve device 27 is opened widely if the working gas is not cooled down, including the start-up period. Therefore, a large amount of flow of the working gas flows between the pulse tube 21.

With a large flow of the working gas between the pulse tube 21 flowing while the low-temperature end of the pulse tube 21 has a high temperature, an amount of work done by the working gas increases, which makes the temperature at the low-temperature end of the pulse tube 21 fall rapidly. Therefore, with the refrigerator 1 according to the present embodiment, it is possible to accelerate cool-down time compared to a conventional refrigerator provided only with an inertance tube capable of flowing an amount of flow only adapted to a specified refrigeration temperature.

However, if a large amount of flow of the working gas such as above is kept flowing, heat loss increases around the vicinity of the joining section of the regenerator 20 and the pulse tube 21. Therefore, refrigeration efficiency of the working gas is reduced before reaching an operating temperature of the working gas.

In the refrigerator 1 according to the present embodiment, as above, the valve device 27 is closed in accordance with the falling temperature of the working gas, which gradually reduces the amount of flow of the working gas between the pulse tube 21 and the buffer tank 29, to reduce the above heat loss gradually. Therefore, with the refrigerator 1 according to the present embodiment, it is possible to accelerate the cool-down time, as well as to securely cool the low-temperature heat exchanger 22 down to the specified refrigeration temperature.

FIG. 3 is a graph demonstrating that the cool-down time can be accelerated by changing the diameter of the flow passage disposed between the pulse tube 21 and the buffer tank 29. In FIG. 3, the vertical axis shows the temperature of the low-temperature end of the pulse tube, and the horizontal axis shows cool-down time.

An arrow A in FIG. 3 designates a characteristic of the refrigerator 1 according to the present embodiment, which is obtained with activating the above controlling procedure under conditions that a ratio of lengths, or an aspect ratio, between the inner diameter and the tube length of the flow passage 24 is set to 0.0055, where the inner diameter is the sum of the inertance tube 25 and the bypass tube 26.

On the other hand, an arrow B in FIG. 3 designates a characteristic of a conventional refrigerator provided only with an inertance tube with a fixed tube and a small aspect ratio of 0.0053.

As shown in FIG. 3, the refrigerator 1 according to the present embodiment (arrow A) has a shorter cool-down time than the conventional refrigerator (arrow B), which demonstrates that the refrigerator 1 according to the present embodiment 1 shortens cool-down time compared to the conventional refrigerator.

In the embodiment above, the flow passage is configured with the inertance tube 25 and bypass tube 26 to shorten cool-down time by controlling the amount of flow through the bypass tube 26. However, the amount of flow of the working gas between the pulse tube 21 and the buffer tank 29 can be controlled also with flow resistance in the flow passage 24.

FIG. 4 shows cool-down time with different aspect ratios (diameter/length) where the inner diameter of the inertance tube 25 is fixed and the tube length is changed. In FIG. 4, an arrow C designates a refrigerator with an aspect ratio of 0.0063, an arrow D designates a refrigerator with an aspect ratio of 0.0054, and an arrow E designates a refrigerator with an aspect ratio of 0.0059.

As shown in FIG. 4, the cool-down time varies with the length of the inertance tube. Therefore, these characteristics can be utilized to configure a refrigerator with shortened cool-down time.

As above, the present invention has been described in detail with reference to preferred embodiments thereof. Further, the present invention is not limited to these embodiments, examples and aspects, but various variations and modifications may be made without departing from the scope of the present invention. 

What is claimed is:
 1. A pulse tube refrigerator comprising: a compressor; a pulse tube including an internal space; a regenerator loaded with a regenerative material for exchanging heat with working gas; a buffer including an internal space with a predetermined capacity; a flow passage to connect an end of the pulse tube and the buffer; a temperature detecting section to detect temperature of the working gas; a flow adjusting section to adjust an amount of flow of the working gas flowing through the flow passage; and a controlling section to control the flow adjusting section in response to the temperature of the working gas detected at the temperature detecting section.
 2. The pulse tube refrigerator as claimed in claim 1, wherein the controlling section controls the flow adjusting section in such a way that the amount of flow of the working gas is reduced if the temperature of the working gas falls, and the amount of flow of the working gas is increased if the temperature of the working gas rises.
 3. The pulse tube refrigerator as claimed in claim 1, wherein the flow passage includes a main flow passage, and a bypass flow passage disposed separately from the main flow passage, provided with the flow adjusting section.
 4. A method of operating the pulse tube refrigerator as claimed in claim 1, comprising: setting the amount of flow of the working gas flowing through the flow passage to a maximum value when the pulse tube refrigerator starts up, and reducing the amount of flow of the working gas flowing through the flow passage in response to the temperature of the working gas falling after the pulse tube refrigerator has started up. 